Ceratopsia.— These were the horned dino saurs, so far as our knowledge goes an exclu sively American group, and although derived doubtless from the armored forms, from what particular type is unknown. They appear sud denly in the West, in rocks of Upper Cretaceous age, and none are transitional but they show at once all of the distinctive features of the group. Chief among these characteristics is a huge head in contrast to the relatively small skull of other dinosaurs. The head bore upon the face a num ber of horns, one, two, or three, while the rear of the skull was prolonged backward to form an expanded bony crest which served to protect the neck and also to provide leverage for the other wise unwieldy head and horns. In the earlier types, such as Monoclonius, the crest was in complete, as it had two large apertures through it, one on either side of the middle line. In the later forms such as Triceratops (Fig. 6), the crest was a complete sheet of bone, although in one rare' genus, Torosanrus, it was still per forate. In the earlier forms, the horn borne on the end of the nose was the larger, those on the forehead not always being in evidence, but in later genera the nasal horn was reduced and the frontal ones were dominant.
The Ceratopsia, like the Stegosauria, were quadrupedal, the weight of armor and armament rendering the ancestral bipedal gait no longer possible, and as certain of the latter .resembled the later mammalian glyptodons, so the former were rhinocerine in general aspect and were doubtless like the rhinoceros in stupid savagery, for broken horns and jaws and punctured crests give abundant evidence of their combativeness.
- Among the dinosaurs there seems to have been a constantly maintained balance throughout their evolutionary• career, for as the carnivores grew and increased in prowess, the herbiVores were forced to meet the' menace of their aggress, simn:irnsev_eral ways,either by increased produc tiveness, or by speed or bulk, or by a partial for saking of the terrestrial habitat. Or they devel oped a defensive armor or aggressive weapons, for one and all lacked brain power; this placed a high premium on brutality, and never perhaps before nor since has the animal world felt to so great an extent the burden of armament.
Extinction.— The cause of dinosaurian ex tinction is by no means clear. One student has argued internecine warfare; another, destructive slaughter of the young, possibly while yet in the egg, by small blood-thirsty mammals; yet an other, climatic change, either diminution of heat, for reptiles are very sensitive to temper ature changes, or decrease of moisture, with a consequent change in the character of vegeta tive life. The close of the age of reptiles was marked by great uplifting or diastrophic move ments over portions of the earth's surface—the so-called Laramide Revolution. As a conse quence the old low-lying dinosaurian habitat was largely restricted, and this, together with other of the necessary chain of sequent events which follow so great a crustal movement, put a period to their existence and cleared the way for the evolution of a potentially higher, though long subject race, the mammals.